Because of incensing environmental concerns worldwide, pulp and paper mills discharge effluents are constantly under scrutiny to ensure that environmental regulations are followed. Because of the high costs involved in the treatment of effluents before their release in the environment, a great deal of research is directed to the modification of current pulp and paper production processes. The research concentrates its efforts in replacing toxic reagents with more environmentally friendly products. A further benefit sought with such changes is that effluents will hopefully require fewer costly conditioning treatments before their release in the environment.
In the various processes proposed in the literature, the oxygen delignification technology is one of the available options towards this direction. Conventionally, oxygen delignification technology uses sodium hydroxide as the alkaline source and the resulting effluent produced can therefore be incorporated into the chemical recovery system of the process for preparing kraft pulps because the same reagent, namely sodium hydroxide, is used, and therefore, there is no reagent interference. On the other hand, the effluent from the sodium hydroxide-based oxygen delignification process (referred to as the O.sub.NaOH technology herein) cannot be sent to the recovery system of the magnesium-based sulphite process because, obviously, the sodium salts are not compatible with the magnesium-based sulphite recovery process. Several publications have therefore concluded that magnesium oxide-based oxygen delignification technology, referred to as O.sub.MgO herein, is preferred for magnesium-based sulphite pulping processes. (see for example Bokstrom et al., Pulp and Paper Canada, 1992, 92 (11), 38; and Luo et al., Tappi Journal, 1992, 75 (6), 183).
Sodium hydroxide has been replaced lately as a base with magnesium oxide (MgO) or magnesium hydroxide (Mg(OH).sub.2) for the oxygen delignification of sulphite pulps. However, because of the low alkalinity of MgO or Mg(OH).sub.2, the temperature of delignification with MgO or Mg(OH).sub.2 must be about 30.degree. C. higher than for the same process using NaOH as the delignification agent (see Luo et al., supra). Alternatively, the delignification rate can be increased in the O.sub.MgO process by the addition of a very limited amount of NaOH, since small concentrations of sodium salts can be tolerated in the recovery system of magnesium-based sulphite process. However, the risk of contamination in the long run is such that this alternative does not represent a desirable selection.
Changing the alkali source in the oxygen delignification process from sodium hydroxide to magnesium oxide or magnesium hydroxide, as taught by Bokstrom et al. supra, decreases the selectivity of lignin to carbohydrate degradation. Moreover, the strength properties also decrease, as illustrated in the relationship between tear index versus tensile index of FIG. 6, by Luo et al. supra. For a given type of wood chips used as starting material, it is well known that sulphite pulps usually have strength properties inferior to that of kraft pulp, and a further decrease in strength properties during the delignification process is therefore unacceptable for commercial operations.
It is known that a post treatment stage with sodium borohydride on an oxidized pulp, such as ozone delignified pulp, leads to increased pulp viscosity. For example, it was reported by Chirat et al. in Holzforschung, 1994, 48 Suppl. 133, that a reduction treatment stage with 0.1% sodium borohydride increases the viscosity of ozone bleached pulp from DP.sub.v of 710 to 920. The chemistry of sodium borohydride reduction is well understood: carbonyl groups present in carbohydrates are reduced to alcohol functionalities (B. Browning, Methods of Wood Chemistry, Vol. 2, P. 685, Interscience Publishers).
In addition, it is proposed by S. Beharic in Papir Dec. 20, 1992, 3(4) pp. 11-15 to add sodium borohydride either before ozone bleaching or after peroxide bleaching to limit the reduction in pulp viscosity. Again, two stages are involved for this pulp treatment.
Accordingly, there is therefore a great need to develop an oxygen delignification process providing pulps with enhanced strength properties and increased viscosity. Preferably, a single stage bleaching process should be considered, wherein a reducing agent would be added in situ. This would represent a significant advance in pulp bleaching, and bring significant benefits to the industry, because the elimination of one treatment stage of pulp represents a significant capital cost reduction.